1. Field of the Invention
The present invention relates to a video transmission system using a communication technology complying with a plurality of communication protocols.
2. Related Background Art
As an apparatus capable of picking up, recording, and reproducing images, there is a camera built-in digital video recorder. As a camera-integrated digital video recorder for consumer use, products for recording and reproducing digital image data of the DV system are known. Such digital image data of the DV system are compressed and encoded using intraframe compression.
On the other hand, as a recording apparatus capable of receiving, recording, and reproducing TV broadcasts, products using the Moving Picture Experts Group (MPEG) system are known, which employ interframe compression as a compression encoding method for digital image data.
Digital image data of the DV system or the MPEG system can be transmitted between different apparatuses with isochronous transfer using an IEEE 1394 serial bus. The isochronous transfer of the DV system is standardized with the AV protocol of the DV transmission standard (IEC 61883, Part 2), and the isochronous transfer in the MPEG system is standardized with the AV protocol of the MPEG transmission standard (IEC 61883, Part 4).
The detail of the camera-integrated digital video recorder for recording and reproducing digital image data of the DV system is standardized with the Standard Definition (SD) format (Part 2) which is included in the DVCR formats (IEC 61834). For example, the SD format defines a configuration for helical-scan recording on a 6.35 mm-wide Mini DV Cassette. The DVCR formats also include standards for the Digital Video Broadcasting (DVB) format (Part 9) and the Advanced Television (ATV) format (Part 10) for recording compressed data of the MPEG system onto a Mini DV Cassette. In other words, the DVCR formats are applicable to a system capable of recording compressed image data both of the DV system and the MPEG system.
The simplest way of dubbing operation using a digital video system composed of a plurality of apparatuses configured on the basis of those DVCR formats is attained by connecting two digital video recorders via a digital interface such as IEEE 1394, to reproduce an original tape of a transmission side while transferring the original tape to a reception apparatus, and to record the received data on another tape of a reception side.
In such a case, in order to record the data of the original tape faithfully at the copy destination, the system of the transmission side must be synchronized with the system of the reception side. Lack of this synchronization may lead to troubles at the reception side such as buffer overflow and recording of data on a location different from the original tape.
The above problems will be specifically described with reference to FIGS. 2A and 2B. FIGS. 2A and 2B both conceptually show how the compressed digital image data are recorded on a tape. FIG. 2A shows a situation of recording at the transmission side, and FIG. 2B shows a situation of recording at the reception side.
In FIG. 2A, reference numeral 1000 denotes an original tape of the transmission side. Reference numeral 1001 denotes a recording track for data recorded on the tape 1000 by a helical-scan head. Laterally arrayed continuous recording tracks as shown in FIG. 2A are formed by conveying the tape 1000. The recording track is formed of areas 1002, 1003 for error correction, and an area 1004 for recording packetized encoded compressed data. Image data, or the like are recorded in the area 1004, with being divided into a plurality of pieces of a predetermined sync block length.
In FIG. 2B, reference numeral 2000 denotes a tape for recording of the reception side. Data to be recorded will be recorded on the tape 2000 in the same manner as in FIG. 2A.
Here, as described above, in the case where the system of the transmission side and the system of the reception side are not synchronized with each other, if the data recorded in the area 1004 at the transmission side are digitally transmitted, then the recording system of the reception side may not operate in time and cause its buffer to overflow (buffer overflow), or on the contrary, may run short of data to be recorded (buffer underflow). This may cause an error of failing to record a predetermined number of data on the tape of the reception side.
Even in the case where the variation of the transmission rate takes place within a range in which no buffer overflow occurs, if the system of the transmission side has a different phase from that of the system of the reception side, then data 1010, for example, in the area 1004 of the transmission side, which are to be recorded at a location 2010 of the reception side, will be recorded at an offset location such as 2011, owing to the influence of the phase shift.
For the DVCR formats, special reproduction is standardized so that the recording may be conducted in a predetermined pattern in consideration of the scan pattern of the head. Displacement of the recorded location of packet data can thus be a fatal recording error.
To avoid such various errors, synchronization should be established at the time of dubbing so that reference clocks may be adjusted between the system of the transmission side and the system of the reception side.
Among the DVCR formats, in the DV system, such synchronization is established by using time stamp information. Since this time stamp information is generated from a predetermined frame change point due to the recorded tape pattern in the reproducing system of the transmission side, the frame change point becomes reference data for the tape location (synchronization is established between the reproducing system and the transmitted data).
On the other hand, the reception side is controlled so that the reference of the recording system is adapted to a change point timing of the frame data reconstructed from the time stamp information (synchronization is established between the received data and the recording system). Therefore, a timing of the system on the transmission side, a timing of the system of the reception side, and a timing of the transmission/reception system for digital data are synchronized with one another, and thereby the tape pattern in the system of the transmission side is faithfully reconstructed as the tape pattern in the system of the reception side.
With regard to the synchronization system in the MPEG system among the DVCR formats, a synchronization signal (corresponding to the time stamp information in the DV system) is communicated in a packet configuration in consideration of asynchronous ATM communication. Specifically, information on the cycle count and cycle offset located in the least significant 25 bits of a source packet header is communicated as the synchronization signal. While the packet location where the synchronization signal is embedded, and the bit length of the synchronization signal are different from those for the DV system, synchronization is established basically in the same way as in the DV system.
However, comparing the DV system with the MPEG system, although the systems have the same basic synchronization algorithm at the time of transmission for dubbing, or the like, the formats are considerably different from each other in the location and bit length of the time stamp information required for synchronization. For example, in the case where the system of the transmission side reproduces a tape for dubbing on which data in the DV system and data in the MPEG system are recorded in a mixed manner, the data in both systems will also be recorded in a mixed manner on another tape in the system of the reception side. This is expected to cause a problem of disturbed synchronization due to the switch of processing for synchronization signals at the time of switching the systems during the dubbing operation.